The present invention relates to a process for preparing ethanebis(alkylphosphinic) acids, and also to the use of the products prepared by this process.
Phosphinic acids and salts of these may be prepared by a variety of methods, and have been described widely within the literature.
Organic phosphinic acids, and their salts and esters are known flame retardants. For example, EP 0 699 708 A1 describes flame-retardant polyester molding compositions, these being rendered flame-retardant by adding the calcium or aluminum salts of phosphinic or diphosphinic acids. The abovementioned salts are obtained by reacting the corresponding phosphonic acids with calcium hydroxide or aluminum hydroxide.
Due to their high phosphorus content and especially their bidentate. nature, the diphosphinic acids are described as highly effective reactive flame retardants for polyesters, e.g. for textile applications. This also applies to ethanebis(methylphosphinic) acid, specifically in the form of its glycol ester (DE 22 36 037 A1).
The preparation of these diphosphinic acids is technically complicated and takes place by an Arbuzov reaction of phosphonous diesters with alkyl dihalides [P. Mastalerz, Rocziniki Chem 38 (1964), pp. 61-64], followed by cleavage of the esters. The phosphonous diesters used are prepared from the corresponding phosphonous dihalides by reaction with alcohols.
Another way of preparing ethanediphosphonic acids is proposed in DE 23 02 523 A1 by reacting alkylphosphonous esters with acetylene and then cleaving the diester with HCI, with formation of alkyl chlorides. Here again, the alkylphosphonous esters used are prepared from the corresponding phosphonous dihalides by hydrolysis and reaction with alcohols.
The abovementioned process has the disadvantage of first requiring inconvenient preparation of the appropriate organic phosphorus compounds. This applies particularly to the esters of alkylphosphonous acids, which in turn are prepared from the corresponding phosphonous dihalides, such as methyldichlorophosphine. Methyldichlorophosphine is prepared by complicated syntheses (Houben-Weyl, Vol. 12/1, pp. 306). In addition, there are byproducts formed which, like some of the above mentioned starting materials, are toxic, or ignite spontaneously, and/or are corrosive, i.e. are highly undesirable.
Another disadvantage is that all of the processes described above for preparing ethanebis(alkylphosphinic) acids have the technically difficult cleavage of the corresponding esters as a final step.
The object on which the invention is based is therefore to provide a process which can prepare ethanebis(alkylphosphinic) acids and avoids the abovementioned disadvantages, and can be carried out in a particularly simple and economic manner, and which gives a high yield of single products. This process should also be clearly superior to the known processes in its effect on the environment. The starting material for the process of the invention is elemental yellow phosphorus, which is easily obtainable, and the process cannot require the complicated cleavage of diphosphinic esters.
This object is achieved by a process for preparing ethanebis(alkylphosphinic) acids by
a) reacting elemental yellow phosphorus with alkyl halides in the presence of alkali metal hydroxide or of alkaline earth metal hydroxide to give a mixture whose main constituents are the alkali metal salts and/or alkaline earth metal salts of alkylphosphonous, phosphorous, and hypophosphordus acid,
b) liberating alkylphosphonous, phosphorous, and hypophosphorous acid by adding mineral acids, and also at the same time precipitating the alkali metal ions and, respectively, alkaline earth metal ions in the form of their salt of the mineral acids, and then
c) esterifying the alkylphosphonous acid from the mixture of the alkylphosphonous, phosphorous, and hypophosphorous acid,
d) isolating the ester of alkylphosphonous acid from the mixture and hydrolyzing the same to give alkylphosphonous acid, and
e) preparing, from the alkylphosphonous acid, the corresponding ethanebis(alkylphosphinic) acid of the general formula (I) 
where R1 and R2 may be identical or different and are hydrogen, a carboxy group, a carboxylic acid derivative, an unsubstituted or substituted alkyl group having from 1 to 10 carbon atoms, phenyl, benzyl, or alkyl-substituted aromatics, and R3 and R4 are identical or different and are an unsubstituted or substituted alkyl group having from 2 to 20 carbon atoms, by free-radical-initiated reaction with alkynes.
Compared with the processes known hitherto, the process of the invention has considerable advantages since it, inter alia, avoids the use of phosphines or phosphonous dihalides as starting materials, produces no halogenated organic byproducts, involves no complicated cleavage of phosphinic esters, and also has a positive balance in relation to product distribution. The process is highly effective and economic to carry out.
The alkyl halides used are preferably methyl chloride or methyl bromide.
The reaction in step a) is preferably carried out in a two-phase system of aqueous alkali metal hydroxide or alkaline earth metal hydroxide or a mixture of these, and an organic solvent.
The organic solvents preferably used in step a) are straight-chain or branched alkanes, alkyl-substituted aromatic solvents, or water-immiscible or only partially water-miscible alcohols or ethers, alone or in combination with one another.
An organic solvent whose use is particularly preferred is toluene, alone or combined with alcohols.
The reaction is preferably carried out in the presence of a phase-transfer catalyst.
The phase-transfer catalyst is preferably a tetraalkylphosphonium halide, triphenylalkylphosphonium halide, or tetraorganylammonium halide.
The temperature during the reaction in step a) is preferably from xe2x88x9220 to +60xc2x0 C.
The temperature is particularly preferably from xe2x88x9210 to +30xc2x0 C.
The reaction is preferably carried out at a pressure of from 0 to 10 bar.
The method of carrying out step a) of the process of the invention is preferably that the yellow phosphorus is suspended in a solvent or solvent mixture and then reacted with alkyl halide and a compound of the formula MOH or Mxe2x80x2(OH)2, or a mixture of these, where M is an alkali metal and Mxe2x80x2 is an alkaline earth metal.
It is preferable for the yellow phosphorus and the alkyl halide to be reacted with one another in a molar ratio of from 1:1 to 1:3, the molar ratio of yellow phosphorus to the compound of the formula MOH or Mxe2x80x2(OH)2 being from 1:1 to 1:5.
Step b) preferably comprises neutralization by addition of a mineral acid.
Step b) preferably comprises neutralization by addition of hydrochloric acid.
The alkali metal salt of the mineral acid and, respectively, alkaline earth metal salt of the mineral acid is preferably precipitated by exchanging the solvent, water, for the alcohol to be used in reaction step c).
The small amounts of phosphines obtained in step b) are preferably removed by oxidation.
The oxidant used preferably comprises hydrogen peroxide.
In step c) the alkylphosphonous acid is preferably esterified directly with a linear or branched alcohol of the general formula Rxe2x80x94OH, where R is a linear or branched alkyl radical having from 1 to 10 carbon atoms.
The alcohol is preferably isobutanol, n-butanol, and/or 2-ethylhexanol.
The alkali metal salt of the mineral acid, or alkaline earth metal salt of the mineral acid, precipitated in step b) is preferably filtered off prior to the esterification process.
One way of esterifying the phosphonous acid to give the corresponding monoester is reaction with higher-boiling alcohols, the water formed being removed by azeotropic distillation.
The alkylphosphonous acid used is preferably methanephosphonous acid.
In step d) the ester of the alkylphosphonous acid is preferably removed by distillation.
In step d) the distilled ester of the alkylphosphonous acid is preferably hydrolyzed with water, and the resultant alcohol is preferably distilled off.
In step e), the alkylphosphonous acid is preferably reacted with an alkyne in the presence of a free-radical initiator.
The free-radical initiators used preferably comprise azo compounds.
The azo compounds are preferably cationic and/or non-cationic azo compounds.
The cationic azo compounds used preferably comprise 2,2xe2x80x2-azobis(2-amidinopropane) dihydrochloride or 2,2xe2x80x2-azobis(N,Nxe2x80x2-dimethyleneisobutyramidine) dihydrochloride.
The non-cationic azo compounds used preferably comprise azobis(isobutyronitrile), 4,4xe2x80x2-azobis(4-cyanopentanoic acid), or 2,2xe2x80x2-azobis(2-methylbutyronitrile).
The free-radical initiators used preferably comprise peroxidic inorganic and/or peroxidic organic free-radical initiators.
The peroxidic inorganic free-radical initiators used preferably comprise hydrogen peroxide, ammonium peroxodisulfate, and/or potassium peroxodisulfate.
The peroxidic organic free-radical initiators used preferably comprise dibenzoyl peroxide, di-tert-butyl peroxide, and/or peracetic acid.
A wide selection of suitable free-radical initiators can be found by way of example in Houben-Weyl, Supplementary volume 20, in the chapter xe2x80x9cPolymerisation durch radikalische Initiierungxe2x80x9d [Free-radical-initiated polymerization] on pages 15-74.
The free-radical initiators are preferably metered in continuously during the reaction.
The free-radical initiators metered in continuously during the reaction are preferably in the form of a solution in the alkyne.
The free-radical initiators metered in continuously during the reaction are preferably in the form of a solution in the solvent used.
To prepare the ethanebis(alkylphosphinic) acids, alkylphosphonous acid obtained after the hydrolysis in step d) is reacted, in the presence of a free-radical initiator, with alkynes of the general formula (II)
R1xe2x88x92Cxe2x89xa1Cxe2x88x92R2xe2x80x83xe2x80x83(II)
where R1 and R2 are identical or different and are hydrogen, a carboxy group, a carboxylic acid derivative, an unsubstituted or substituted alkyl group having from 1 to 10 carbon atoms, phenyl, benzyl, or alkyl-substituted aromatics.
The alkynes used may be either the unsubstituted alkyne where R1 and R2=H in formula (II), singly substituted derivatives where R1=H and R2 xe2x89xa0H in formula (II), or else doubly substituted alkynes where R1 and R2xe2x89xa0H in formula (II).
Examples of these alkynes are ethyne, phenylacetylene, diphenylacetylene, propyne, 1-butyne, 2-butyne, 1-phenylbutyne, 1-pentyne, 2-pentyne, 1-phenyl-1-pentyne, 1-hexyne, 2-hexyne, 3-hexyne, 1-phenyl-1-hexyne, 1-heptyne, 1-octyne, 4-octyne, 1-nonyne, 1-decyne, 1-dodecyne, the alkynols propargyl alcohol, 1-butyn-3-ol, 2-butyn-1-ol, 2-butyne-1,4-diol, 1-pentyn-3-ol, 2-pentyn-1-ol, 4-pentyn-1-ol, 4-pentyn-2-ol, 3-hexyn-1-ol, 5-hexyn-1-ol, 3-hexyne-2,5-diol, 2-octyn-1-ol, 1-octyn-3-ol, 3-nonyn-1-ol, 3-decyn-1-ol, and also propargyl chloride, propargyl bromide, propargylamine, propiolic acid, methyl propiolate, ethyl propiolate, 2-butynoic acid, ethyl 2-butynoate, 4-pentynoic acid, 5-hexynonitrile, 2-octynoic acid, methyl 2-octynoate, methyl 2-nonynoate, acetylenedicarboxylic acid, diethyl acetylenedicarboxylate, and dimethyl acetylenedicarboxylate.
Preferred alkynes are the 1-alkynes, propargyl alcohol, butynediol, propiolic acid, and acetylenedicarboxylic acid derivatives. Particular preference is given to the use of ethyne(acetylene).
The reaction preferably takes place at a temperature of from 40 to 200xc2x0 C.
The reaction particularly preferably takes place at a temperature of from 70 to 130xc2x0 C.
The reaction preferably takes place in the presence of a solvent.
The reaction preferably takes place in acetic acid as solvent.
The reaction preferably takes place by introducing gaseous acetylene (ethyne) at atmospheric pressure.
The reaction preferably takes place at superatmospheric pressure.
The manner of conducting the process is preferably such that after partial conversion the precipitating ethanebis(alkylphosphinic) acid is filtered off, and further alkyne is added after replacing the alkylphosphonous acid consumed.
The present invention also provides a process in which yellow phosphorus is reacted with methyl chloride in the presence of sodium hydroxide solution and of the phase-transfer catalyst tributylhexadecylphosphonium bromide, to give the sodium salt of the methylphosphonous acid, and the free acid is liberated from this by adding hydrochloric acid, and is esterified with 2-ethylhexanol in the mixture, the ester is isolated by distillation and hydrolyzed, and the resultant pure methanephosphonous acid is reacted with acetylene (ethyne) in the presence of a cationic or non-cationic free-radical initiator or in the presence of a peroxidic free-radical initiator, to give ethanebis(methylphosphinic) acid.
The present invention also provides a process in which yellow phosphorus is reacted with methyl chloride in the presence of sodium hydroxide solution and of the phase-transfer catalyst tributylhexadecylphosphonium bromide, to give the sodium salt of the methylphosphonous acid, and the free acid is liberated from this by adding hydrochloric acid, and is esterified with 2-ethylhexanol in the mixture, the ester is isolated by distillation and hydrolyzed, and the resultant pure methanephosphonous acid is reacted with acetylene (ethyne) in the presence of a cationic or non-cationic free-radical initiator or in the presence of a peroxidic free-radical initiator in acetic acid, to give ethanebis(methylphosphinic) acid, and this is continuously removed from the reaction mixture by a circulating filter system, and the methanephosphonous acid consumed is likewise continuously replaced by fresh acid.
The desired diphosphinic acids are obtained with high selectivity and high purity.
Either alkylphosphonous acids or the alkynes may be used in excess, since the reaction partners always react in a molar ratio of 2 to 1 (alkylphosphonous acid to alkyne).
The invention also provides the use of the ethanebis(alkylphosphinic) acids prepared by the process of the invention as starting materials for preparing flame retardants for polymers.
The invention further provides the use of the ethanebis(alkylphosphinic) acids prepared by the process of the invention as starting materials for preparing flame retardants for thermoplastic polymers, such as polyethylene terephthalate, polybutylene terephthalate, or polyamide.
The invention also provides the use of the ethanebis(alkylphosphinic) acids prepared by the process of the invention as starting material for preparing flame retardants for thermoset resins, such as unsaturated polyester resins, epoxy resins, polyurethanes, or acrylates.
Finally, the invention also provides the use of the ethanebis(alkylphosphinic) acids prepared by the process of the invention as precursors for the chemical synthesis of other phosphorus-containing compounds.